1. Field of the Invention
This invention is directed to a stabilized imageable coating composition useful in the preparation of printing plate precursors. More particularly, the coating composition includes ionomeric polymers useful in thermally switchable printing plates, and associative thickeners that prevent flocculation and agglomeration of aqueous carbon dispersions contained in the coating composition.
2. Background Information
Ionomeric polymers are of interest for xe2x80x9cno-processxe2x80x9d thermal printing plates because they provide a hydrophilic background that can be converted by IR imaging into oleophilic areas that take ink on press. However, ionomeric polymers used in thermally xe2x80x9cswitchablexe2x80x9d printing plates cause aqueous carbon dispersions with ionic stabilization to flocculate when mixed in a coating formulation or to agglomerate during drying of the coating. Stability to flocculation results in improved optical density and is therefore desirable.
The problems encountered by the use of ionically stabilized aqueous carbon dispersions have been described in the literature. For example, WO 00/63025 discloses the use of aqueous polymer-grafted carbon dispersions as a solution to the flocculation problem. Polymer-grafted carbon is much more resistant to flocculation. However, polymer-grafted carbon can have a detrimental effect on the background sensitivity of printing plates and on blanket toning, depending upon the choice of ionomeric polymer and on the type of ink used. Polymer-grafted carbon dispersions are currently available only on an experimental basis and are expected to be expensive once commercialized.
U.S. Pat. No. 6,187,085 discloses a carbon black printing ink having Newtonian flow with improved rheological properties. However, the disclosed ink compositions are all in mineral oil, and thus could not be used in aqueous compositions. Furthermore, while the polyisobutylene succinimide used together with sodium hydroxide may form a sodium salt in situ, the resulting polymer has only an ionic terminus and thus is not an ionomer. Triethylene glycol is also required as a rheological additive.
European Patent No. 0 112 465 discloses an ink composition based on a novel alkyl binder which improves ability to de-ink paper for recycling. However, all compositions are in mineral oil, and thus incompatible with aqueous formulations. No mention is made of ionomeric polymers.
U.S. Pat. No. 5,312,704 discloses a composition of particles of pigment and a resin or blend of resins with good Theological properties. The composition is directed to use in toners. The solid toner particles disclosed in this reference are not compatible with aqueous formulations. No associative thickeners are discussed.
U.S. Pat. No. 4,569,965 discloses crotonate copolymers and their use as thickeners providing high viscosity at high shear rates in aqueous systems and as sizing agents. These copolymers are directed to use as paper printing pastes, and are not directed to uses in printing plates. Furthermore, the use of thickeners with aqueous carbon dispersions or with ionomeric polymers is not discussed.
J. C. Padget in xe2x80x9cPolymers for water-based coatingsxe2x80x94a systematic overviewxe2x80x9d, J. Coat. Technol. Vol. 66 No. 839, pp. 89-105, December 1994, has mentioned the advantage of associative thickeners in minimizing flocculation of pigments in aqueous coatings. However, the article is directed to paint or architectural coatings, and does not address coatings for printing plates.
U.S. patent application Ser. No. 09/688402, now abandoned, discloses the use of thixotropic additives to water-soluble film-forming polymers to improve certain properties of coating compositions. However, this application does not address the stabilization of aqueous carbon dispersions, or the use of thixotropic additives with ionomeric polymers.
There is therefore a need in the art for a composition and method to increase the stability to flocculation of aqueous carbon dispersions in ionomeric polymers for coating compositions to be used to prepare printing plate precursors.
We have found that aqueous carbon dispersions with ionic surface treatments may be used in ionomeric switchable polymer coatings without the occurrence of flocculation if used in conjunction with associative thickeners. Particularly useful are rheology modifiers or polymers that exhibit pseudoplastic behavior in solution. These materials provide viscosity to stabilize the carbon particles, but undergo shear thinning to minimize coating problems. We have also found that such rheology modifiers may be used to stabilize low viscosity aqueous carbon formulations in which even polymer-grafted carbon dispersions are inadequately stable. Printing plates prepared from aqueous carbon formulations incorporating polymers that exhibit psueoplastic behavior have been found to deliver darker prints at lower exposure energies than do plates prepared without these polymeric additives, that is, the plates of the present invention have faster photospeeds.
In one aspect, the invention relates to a printing plate coating composition comprising: (a) a carbon pigment dispersible in water, (b) at least one polymer comprising at least one ionomeric polymer subunit, and (c) at least one high molecular weight thickening agent which shows pseudoplastic behavior.
In another aspect, the invention relates to a printing plate precursor comprising a coating obtainable by applying the coating composition on a suitable printing plate substrate. As used herein, the term xe2x80x9cprinting plate substratexe2x80x9d refers to a substrate which is capable of being mounted on a printing press for lithographic duplication or printing. The precursor may be prepared, for example, by applying the coating composition by means of a slot coater or roll coater.
The present invention is useful in the production of printing plates that make use of aqueous carbon-pigmented coatings. In particular, the invention enhances the use of coating compositions containing ionic switchable polymers with ionically stabilized aqueous carbon dispersions. The invention is also useful in that it makes practical the use of ionically stabilized commercially available aqueous carbon dispersions in aqueous ionomeric coating formulations. The invention also makes possible the coating of low viscosity aqueous formulations containing either polymer-grafted or ionically-stabilized carbon dispersions.
The present invention has the advantage that polymeric additives that provide rheological control make it possible to use relatively inexpensive, commercially available aqueous carbon dispersions in coating compositions for printing plates. In particular, the present invention makes possible the use of ionic functionalized carbon dispersions in switchable printing plates to afford improved performance in the background or non-image areas.